Flavius Placidius Valentinianus (2 July 419 – 16 March 455), commonly known as Valentinian III, was Western Roman Emperor from 425 to 455.
Valentinian was born in the western capital of Ravenna, the only son of Galla Placidia and Flavius Constantius. The former was the younger half-sister of the western emperor Honorius, and the latter was at the time Patrician and the power behind the throne.
Through his mother, Valentinian was a descendent both of Theodosius I, who was his maternal grandfather, and of Valentinian I, who was the father of his maternal grandmother. It was also through his mother's side of the family that he was the nephew of Honorius and first cousin to Theodosius II (the son of Honorius' brother Arcadius), who was eastern emperor for most of Valentianian's life. Valentinian had a full sister, Justa Grata Honoria, who was probably born in 417 or 418 (the history of Paul the Deacon mentions her first when mentioning the children of the marriage, suggesting she was the eldest). His mother had previously been married to Ataulf of the Visigoths, and had borne a son, Theodosius, in Barcelona in 414; but the child had died early in the following year, thus eliminating an opportunity for a Romano-Visigothic line.
When Valentinian was less than two years old, Honorius appointed Constantius co-emperor, a dignity he would hold until his death seven months later. As a result of all these family ties, Valentinian was the son, grandson, great-grandson, cousin, and nephew (twice over) of Roman Emperors.
After the death of his father in 421, Valentinian followed his mother and his sister (Justa Grata Honoria) to Constantinople, when Galla Placidia broke with her half-brother, Emperor Honorius, and went to live at the court of Theodosius II.
In 423, Honorius died, and the usurper Joannes took the power in Rome. To counter this threat to his power, Theodosius nominated Valentinian Caesar of the west (October 23, 424), and betrothed him to his own daughter Licinia Eudoxia (Valentinian would marry her in 437). In 425, after Joannes had been defeated in war, Valentinian was installed Western Emperor in Rome, on October 23, at the age of six.
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